The present invention relates to resistance-heated containers or crucibles, commonly called boats, heaters, or intermetallic sources, for use in the vacuum vaporization of metals.
It is well known in the prior art that various substrates, for example, glass, metal and plastics, may be coated with a thin layer of metal by vacuum deposition processes. Generally, in such processes, a vaporizable metal, such as aluminum, chromium, copper, zinc, silver or gold, is vaporized at a high temperature under vacuum and the metallic vapor deposited on the desired substrate. The metal is vaporized by heating in a boat, which serves both as a heater and a crucible. Typically, a boat consists of an elongated bar of electrically conductive material. Boats may have flat vaporization surfaces or, more usually, a depression in the vaporization surface to hold a supply, or pool, of metal to be vaporized. The boat is connected to a source of electricity. The electrical resistance of the boat causes the boat to heat upon passage of electrical current therethrough. The heat vaporizes the metal supplied to the vaporization surface.
The resistance-heated vaporization boats commonly in use today consist of a mixture of ceramic or intermetallic composite materials, for example, titanium diboride with boron nitride, or silicon carbide, or titanium diboride with boron nitride and aluminum nitride. Such mixtures have service lives substantially longer than the metal or graphite materials previously used. However, these materials have serious drawbacks in use. The mixtures are selected on the basis of their chemical and physical resistance to the metal being vaporized and their appropriate electrical resistance. Typically, boride components are utilized to impart high electrical conductivity, physical and chemical resistance to attack by molten and vaporized metals and to provide wettability to the vaporization surface. Typically, nitride components, which are good electrical insulators but are, generally, not wet by molten metals, such as aluminum, are utilized to adjust the electrical resistivity to a level for heating element use. Thus, the materials in use today are compromises which do not provide the best characteristics for either purpose.
Metals vaporized in conventional boats act as a parallel conductor in the electrical circuit containing the boat. The electrical resistance of the circuit is dependent upon the metal depth, composition and temperature. At high temperatures, such as those used in metal vaporization processes, the metals and impurities in the metals react with the materials of the boat varying the electrical resistance of the boat. This variance requires continual adjustment of the power supply to maintain a desired power input, hence, the boat temperature, and, in turn, the vaporization rate.
The present vacuum metallizing boats overcome the shortcomings of the prior art by providing a multi-layered boat. The present boat allows the vaporization surface of the boat to be fabricated of materials selected solely on the basis of their wettability, their chemical and physical resistance to molten metals and metal vapors and independently allows a heating element portion to be selected from materials solely on the basis of their electrical resistance properties.